This invention relates to a deformation measuring method and device, and more particularly to an improvement of deformation measuring method and device in which a part of the surface of an object is irradiated with a laser beam before and after being deformed, to obtain speckle patterns. The amount of deformation of the object is determined on the basis of the cross-correlation function between the speckle patterns.
A speckle pattern is formed by interference of diffusion lights which are reflected from a rough surface of an object when a laser beam is applied to the surface. When the surface is displaced or deformed, the speckle pattern is displaced while gradually deformed. In this connection, a "speckle correlation method" has been proposed, in the art in which a speckle pattern is photoelectrically scanned and the speckle displacement is obtained from the correlative peak positions of the signals thus obtained, and the relations between the speckle displacement and the displacement (or deformation) of the surface are utilized to measure the minute deformation of the object due to translation, rotation and distortion or the like. This method is disclosed in detail in Japanese Patent Publication No. 52963/1984; "Laser Science Research" No. 6, pp 152-154 (1984) and "Latest Precision Measurement Technology" pp 241-244, on July 1, 1987.
Of the speckle correlation methods, the most practical method uses a one-dimensional image sensor 15 and a micro-computer 16 as shown in FIG. 1. By this method, a parallel movement of 1 .mu.m or more and a rotation of the order of 10.sup.-5 rad can be measured.
In the device as shown in FIG. 1, a laser beam about 1 mm in diameter which is generated by a laser source 12 is applied to a measuring point on an object, through a magnifying lens 14 if necessary, and the one-dimensional image sensor 15 is disposed in the propagating passage of the light beam reflected from the measuring point. In this case, the beam diameter W on the object 10 and the distance Lo between the object 10 and the image sensor 15 are adjusted so that an average diameter of the speckle pattern is approximately .lambda.Lo/W (.lambda.: wavelength of the laser beam) on the sensor 15 is larger than the pitch (10 to 20 .mu.m) of the sensor. In addition, the axis of the one-dimensional image sensor 15 is adjusted so as to be coincided with the direction of displacement of the speckle pattern which is determined by the optical system and the kind of displacement (the direction of parallel movement, rotation or distortion) of the object.
The output of the one-dimensional image sensor 15 is subjected to A-D (analog-to-digital) conversion and applied to a micro-computer 16. A correlation unit 18 calculates a cross-correlation function between the outputs of the micro-computer which correspond to the speckle patterns before and after the deformation of the object, and the speckle displacement is obtained from the peak positions of the correlation function substantially in real time. In this connection, in order to reduce the time required for calculation of the cross-correlation function, a method of calculating a "characteristic correlation" has been proposed in the art. In this method, the output signals of the one-dimensional image sensor 15 are binary-coded with respect of the average thereof. The speckle pattern thus obtained has high contrast, so that the peak position is coincided with that of the ordinary cross-correlation function at all times. Accordingly, the speckle displacement can be detected from the extreme position of the cross-correlation function.
In the conventional speckle correlation method, the cross-correlation function is obtained as follows: The speckle pattern provided by a scanning operation before the deformation of the object is used as a fixed reference speckle pattern (data) and the speckle pattern provided by a scanning operation while the object is being deformed is utilized as a comparison speckle pattern (data). Those data are compared with each other to obtain the cross-correlation function therebetween. Alternatively, the speckle pattern provided by a scanning operation while the object is being deformed is employed as a comparison data, but the data obtained by the preceding scanning operation, which is carried out immediately before the present scanning operation for the comparison data, is used as a reference data. That is, the cross-correlation function is obtained while the reference data is renewed every time.
In the former method adopting the fixed reference data, the speckle pattern to be compared is largely changed with the deformation of the object in comparison with the reference speckle pattern, and therefore the extreme value of the cross-correlation function becomes lower than the unrelated peak values around it, as a result of which it is impossible to obtain the position of the extreme value correctly, and the range of measurement is limited.
On the other hand, in the latter method adapting the reference data to be renewed every time, in the case where a speckle pattern is displaced at a distance less than half of the pitch interval of the one-dimensional image sensor 15 between two successive scanning operations (the present and preceding scanning operations), the reference data is identical to the comparison data, so that the position of the extreme value is not moved and the displacement of the object is disregarded. Since this error occurs every scanning operation, the low speed displacement of the speckle pattern cannot be detected particularly when the displacement between two successive scanning operations is less than half of the pitch interval of the image sensor.